Hydration and Exercise, Part 2

First off I want to apologize to you for the big gap since my last post. A week ago our home was burglarized and among all of the stuff stolen were our computers. That put me out of business for a few days. But I’m back up to speed now so let’s get back to the topic of hydration, or as coach Gordo Byrn calls it, “managed dehydration.”

I think coach Byrn’s two-word summary is an appropriate mindset as you read what I’ve got to say here and in Part 3. I suspect you may not agree with all of this. I would certainly understand as I also resisted the evidence I had seen over the past several years in favor of what I had been told by so many experts including scientists. This post may turn your world upside down, but bear with me as we work through this. Change is never easy.

Over the last few weeks I’ve been rethinking what I understand and believe about hydration during exercise. In a single sentence, here is what I’ve come to believe endurance athletes should do during exercise in regards to hydration: Drink when thirsty; don’t drink when not thirsty. For shorter endurance events – those lasting less than, let’s say, four hours – that’s a not a huge change. And the shorter the event the more likely the athlete is to drink to thirst any way.

But that suggestion flies in the face of what I’ve suggested to the athletes I’ve coached in long endurance events such as Ironman triathlons and marathons. For at least 20 years I’ve told them they needed to have a hydration schedule during such events and stick with it, even setting their watch to beep every 10 minutes or so to remind them to drink. This was based on the oft-repeated axiom that when you become thirsty it’s too late. The research does not generally support this notion [1,2] with the possible exception of older athletes [3,4,5] although not all of the research agrees on this age issue [6]. Generally, when left to their own devices, a thirsty athlete will drink sufficiently to maintain an adequate level of hydration without negative consequences for performance. The key is paying attention to your thirst.

I’ve also said that for long events completed in the neighborhood of about 12 hours or less that their hydration source should also be their refueling source. Basically, that means they needed to use a sports drink for both water and calories. As the finishing time gets longer than 12 hours I’ve suggested they use more solid foods and wash them down with water and sports drinks. And, I said, nearly all long-endurance athletes should be supplementing with sodium even though their sports drinks probably already have some in it.

Now I’ve changed my mind on all of this. It hasn’t been easy. I’ll try to explain.

There is one bit of information I had been unable to account for given my former position on hydration. It had been staring me in the face but I’d ignored it. This has to do with the body’s sodium concentration. During exercise, as fluid is lost through sweating and in other ways, the concentration of sodium in the body actually increases. The reason is because much more fluid is lost than sodium. One might lose around a liter of water during exercise but only lose a small amount of sodium in sweat. Normal body sodium levels are about 140 millimoles per liter (mmol/l) of water while sweat is about 20 to 60mmol/l.

So let’s say an average-sized human body contains 40 liters of water when at rest and normally hydrated. That means it has stored away something like 5600mmol of sodium (40 x 140 = 5600). If one liter of fluid is lost during exercise and with that 60mm of sodium are excreted (the high end, or “salty” sweater) then the new sodium concentration is about 142mmol/l (5600 – 60 = 5540 / 39 = 142.05). The concentration of sodium has risen, not declined. Guess what happens next after a sufficiently large rise in sodium concentration occurs – your thirst mechanism kicks in and you drink water to dilute the sodium bringing it back down to something closer to 140mmol/l. One study found that a rise of about 2-3% of plasma sodium concentration evoked a strong desire to drink [7].

So your sodium becomes more concentrated during exercise as you sweat, not less as we’ve been led to believe. In other words, you don’t need to replace lost sodium during exercise because the loss is inconsequential while the volume of water lost is significant. But even if you did, the sodium content of most sports drinks is 10-25mmol/l, not enough to replace the loss (unless you overhydrate which raises the specter of hyponatremia - more on that shortly). More than about 25mmol/L of sodium makes the drink unpalatable. The extracellular fluid in your body, where much of the sodium is stored, is about the concentration of sea water. If you’ve ever swallowed sea water you know how bad that would be to drink. So should you therefore supplement with sodium capsules during an ultra event? I’ll get at that in Part 3.

Would not taking in sodium during a long race or workout impact your performance? Not according to the research. Adding sodium to a sports drink did not improve performance in a time trial effort after four hours of exercise at a moderate intensity [8]. Nor did sodium in a sports drink impact the ability to complete six hours of moderate-intensity exercise [9].

The greater issue for the long-endurance athlete is hyponatremia. This was discussed in Part 1 of this series (see Hydration, Part 1). The condition of hyponatremia is considered to be a sodium concentration level of less than 128mmol/l by some. As you can see from the example above the only way to get there would be to drink more water than is lost during exercise. That would dilute your sodium stores. So the issue is not replacing sodium but rather not drinking too much fluid. Thirst is the key to this balance. If you drink only when thirsty and to a level which satisfies thirst then you will not drink too much.

I know I haven’t yet answered the questions at the end of Part 1 in this series on hydration, but I’m going to stop here and let this sink in. In Part 3 I’ll get at how I now deal with those questions for the athletes I coach.

References
1. Maresh, C.M., et al. “Effect of Hydration Status on Thirst, Drinking, and Related Hormonal Responses During Low-Intensity Exercise in the Heat.” Journal of Applied Physiology 97, No. 1 (2004): 39-44.
2. Schroeder, J.M., et al. “A Comparison of Three Fluid Replacement Strategies for Maintaining Euhydration During Prolonged Exercise.” Canadian Journal of Applied Physiology 22, No. 1 (1997): 48-57.
3. Leaf, A. “Dehydration in Elderly.” New England Journal of Medicine 311 (1984): 791-792.
4. Marszalek, A. “Thirst and Work Capacity of Older People in a Hot Environment.” International Journal of Occupational Safety and Ergonomics No. 135 (2000): 135-142.
5. Ainslie, P.N., et al. “Energy Balance, Metabolism, Hydration, and Performance During Strenuous Hill Walking: The Effect of Age.” Journal of Applied Physiology 93, No. 2 (2002): 714-723.
6. Bossingham, M.J., et al. “Water Balance, Hydration Status, and Fat-Free Mass Hydration in Younger and Older Adults.” American Journal of Clinical Nutrition 81, No. 6 (2005): 1342-1350.
7. Hubbard, R.W., et al. “Influence of Thirst and Fluid Palatability on Fluid Ingestion.” In Gisolfi, C.V., D.R. Lamb, eds. “Perspectives in Exercise Science and Sports Medicine.” Vol. 3. Fluid Homeostasis During Exercise. Indianapolis, IN: Benchmark Press (1990): T96.
8. Merson, S.J., et al. “Rehydration with Drinks Differing in Sodium Concentration and Recovery from Moderate Exercise-Induced Hypohydration in Man.” European Journal of Physiology 103, No. 5 (2008): 585-594.
9. Barr, S.I., et al. “Fluid Replacement During Prolonged Exercise: Effects of Water, Saline, or No Fluid.” Medicine and Science in Sports and Exercise 23 (1991): 811-817.